Physics is the name given in game programing to the ability of graphic objects to react to gravity and other forces on a tablet screen. I think it's key to making apps in the future... and today I just downloaded two great programs to add physics and animation to an e-book app.
The two great programs are TexturePacker and PhysicsEditor. They allow an artist to create their own physics and animation to add to their books. They're from Code'n'Web. So that sounds like my wintertime hobby to figure out how they work. I think it should make for some awesome potential.